The L4.1.15_2.1.0_LINUX BSP Release supports only Qt5.6 only, as you mentioned.

If you wish to use a newer version of Qt you would need to port it. There may be yocto layers with Qt 5.8 packages but the porting requires reviewing dependencies etc. You would need to consult Qt documentation regarding this.

I haven’t personally tested Qt 5.8 but an alternative would be Qt 5.8 for device creation. The official documentation does mention i.MX6 boards so perhaps it would be a good starting point.